Hi,
I studied a paper about simulation of an turbine by using Fluent and I saw the following sentence and I don't have any information about that.
"All converged solutions had a mass imbalance of less than 0.3%"
In Fluent, how i can set above sentence?
I am grateful that guide me about this cases.
Thanks
Suppose in your domain you have one inlet and one outlet. Since you are solving physical equations, you need to conserve the mass (First law - Mass is conserved).
Now in a computer, the physical equation is converted into a numerical equation, and while solving there may be errors cropping up. Such as truncation error, numerical error etc. Because of this, the mass going in at inlet and mass coming out from inlet will not be exactly same, there will be a slight difference. This difference is called mass imbalance. But physics says mass should be conserved. So while using any numerical scheme, the accuracy of the solution is achieved when this mass imbalance is reduced to very low level. This is what people specify whenever they are presenting any mathematical solution
You cannot set the mass imbalance, it is a result of the simulation.
In fluent you can calculate it at post processing time, and you can also monitor it during the iteration to check convergence.
Suppose in your domain you have one inlet and one outlet. Since you are solving physical equations, you need to conserve the mass (First law - Mass is conserved).
Now in a computer, the physical equation is converted into a numerical equation, and while solving there may be errors cropping up. Such as truncation error, numerical error etc. Because of this, the mass going in at inlet and mass coming out from inlet will not be exactly same, there will be a slight difference. This difference is called mass imbalance. But physics says mass should be conserved. So while using any numerical scheme, the accuracy of the solution is achieved when this mass imbalance is reduced to very low level. This is what people specify whenever they are presenting any mathematical solution
Since natural quantities like mass, energy etc. are conserved under their respective appropriate conditions, mass imbalance shouldn't exist neither in theoretical nor in numerical studies.
In CFD solution like the aforementioned, most probably author's intention was to point out the accuracy of his computations according to the boundary conditions set or the selected solver. In this case, I believe that the solution didn't converge mainly due to selected CFD conditions that were not optimised for that kind of problem. Nevertheless, in such cases the reader should keep in mind that once the author's methodology will be followed, there will be a 'mass imbalance' or better a 'miss-conversion' at the final results.
Hi,
You can calculate the mass imbalance by going to > Report > you choice reports >Flux and you click calculate (you can choose between which boundaries you want to calculate it).
Good luck.
there i a way to monitor the mass imbalance during fluent calculation?
Hi,
Giosue, you can set a Solution Data Export (Calcularion Activities > Automatic Export > Create > Solution Data Export ) There you select a type of file (I recommend ASCII), the surface and the quatity you want to calculate in each time step/flow time for all the domain. It return a text file for each time step, then you can analyse the data in matlab or similar.
Good luck,
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